Lecture 30 Flashcards
How are heavy and light chain gene families encoded
On separate chromosomes
What is somatic recombination
The process by which B cell segments are rearranged to create different possible antibodies
Tightly regulated
Many of the same proteins involved in DNA repair
Genes in Light chain
V, J, C
Variable region, Joining region, and Constant region genes
Genes in Heavy chain
V, D, J, C
Variable region, Diversity region, Joining region, Constant region gene
D–>only in Heavy chains
Where are CDR 1 and 2 encoded
In V segments of light and heavy chains
Both V regions are in germline DNA
Where is CDR3 encoded in heavy and light chains
Heavy chain–>V, D, J gene segment
Light chain–> V-J segment
Types of light chains
Kappa and lambda chains
Only 1 chains will be expressed while silencing the other
2 Loci
How many loci in Heavy chain
1
Different constant regions represent different isotypes
Overall pathway for somatic recombination
Inherited stem cell (B cell precursor)–>somatic recombination–>recombinase proteins join separate gene segments together–>mature naive B cell
Irreversible
How many V regions to chose from
46
How many D regions to chose from
23
How many J regions to chose from
6
What is combinatorial diversity
Diversity that results in different combinations of V, D, and J regions
What do Recombination signal sequences (RSSs) do
RSSs flank each antibody gene segment to then be recognized be Recombinase enzymes
What does each Recombination signal sequence (RSS) contain
Nonamer (9bp) and heptamer (7bp) sequence with either a 12 OR 23 bp spacer
What is the 12/23 Rule
That 12bp RSS MUST pair with a 23bp RSS for recombination to occur
What pairing does RSS direct in light and heavy chains
Light chain–> V-J
Heavy chain–> D-J and V-DJ
What is within the Loop of DNA formed after RSS regions are brought together
Segments that were not selected
In loop–>out of segment
Function of RAG-1 and RAG-2
RAG= recombination activating gene
Recognizing and cutting DNA at the immunoglobulin-encoding region and the RSS
Necessary for recombination
Structure of RAG
Covalently closed DNA hairpin ends
What is the signal joint
Loop of DNA excised when RSS are brought together
No longer on chromosome–>deleted
What is the coding joint
Coding region of selected V and J regions that remain (not in loop)
What is junctional diversity
When nucleotides are added or removed at junctions between V-D and D-J (or V-J for the light chain)
Occurs during recombination
What is Artemis
Endonuclease that opens the DNA hairpins during junctional diversity
Main mechanism of Junctional Diversity
Hairpin cleavage
Ways that a hairpin can be opened (cleaved)
1) Exonuclease activity to remove nucleotides on each side of coding gene
2) Addition of palindromic (P) nucleotides at overhangs
3) Terminal Deoxynucleotidyl Transferase (TdT), can add up to 20 N-nucleotides (non-template encoded) to cleaved stranded mainly in heavy chain
Function of repair enzymes in Junctional diversity
Trim off non-matching nucleotides,
Fill in remaining single-stranded gaps
Ligate new DNA
